1. Field of the Invention
The present invention relates to a charging circuit for a capacitor, and more particularly, to a charging circuit which utilizes an inversion capacitance and a depletion capacitance to determine a current ratio for charging a capacitor.
2. Description of the Prior Art
Generally, the Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is formed from top to bottom as a metal layer, which is currently replaced by Polycrystalline Silicon, an oxide layer and a semiconductor layer (i.e. P-type or N-type Semiconductor) to form structurally a MOS transistor capacitor. The silicon dioxide (SiO2) is usually utilized for the oxide layer to form a dielectric material of the capacitor. Thickness of the oxide layer and the dielectric constant of silicon dioxide are further utilized to determine capacitance of the capacitor. Last, the MOS transistor capacitor has the polycrystalline silicon as a gate and the semiconductor layer as abase to form two terminal ends for connection.
Please refer to FIG. 1A, FIG. 1B and FIG. 2, wherein FIG. 1A illustrates a schematic diagram of a conventional MOS transistor capacitor MOS_C for charging, FIG. 1B illustrates a schematic diagram of different operational conditions of the MOS transistor capacitor MOS_C versus different capacitances, and FIG. 2 illustrates a schematic diagram of a terminal voltage VC1 of the MOS capacitor MOS_C at different timings. As shown in FIG. 1A and FIG. 1B, the MOS transistor capacitor MOS_C utilizes a stable current source CS for continuously charging. Due to an increasing voltage value of a gate of the MOS transistor capacitor MOS_C, the MOS transistor capacitor MOS_C switches its capacitances between a depletion capacitance C_del or an inversion capacitance C_inv, which correspond to different operational conditions. Please refer to FIG. 2. Since the MOS transistor capacitor MOS_C can be either the depletion capacitance C_del or the inversion capacitance C_inv, the terminal voltage VC1 of the MOS transistor capacitor MOS_C corresponds to two lines with different slopes at the threshold voltage Vth. Under such circumstances, the capacitance of the MOS transistor capacitor MOS_C has dramatically changeable capacitances at the threshold voltage Vth. Therefore, it has become an important issue in the art to provide a charging circuit for a capacitor to avoid dramatically changeable or discontinuous capacitances of the MOS transistor capacitor MOS_C at the threshold voltage Vth.